Specifications

Version 1.1 rev Jan 2013
Page 78
iXon Ultra
, Features and Functionality
3.7 - COUNT CONVERT
One of the distinctive features of the iXon Ultra is the capability to quantitatively capture and present data in units of
electrons or photons; the conversion applied either in real time or as a post-conversion step. Photons that are incident
on pixels of an array detector are captured and converted to electrons. During a given exposure time, the signal in
electrons that is collected in each pixel is proportional to the signal intensity. In EMCCDs, the signal in electrons
is further multiplied in the EM Gain register. The average multiplication factor is selected in the software from the
RealGain™ scale. It can be desirable to directly quantify signal intensity either in terms of electrons per pixel, or in terms
of incident photons per pixel. However, during the readout process, array detectors must rst convert the signal in
electrons (the multiplied signal in the case of EMCCDs) into a voltage, which is then digitized by an Analogue to Digital
Converter (ADC). Each Analogue to Digital Unit (ADU) is presented as a ‘count’ in the signal intensity scale, each count
corresponding to an exact number of electrons. Furthermore, the signal value in counts sits on top of an electronic bias
offset value. In the iXon Ultra this ‘baseline’ is clamped at 200 counts in normal operation modes.
Therefore, in order to calculate to the original signal in electrons, the electron to ADU conversion factor must be very
accurately stored by the camera (which varies depending on the pre-amplier gain selection chosen through software).
Calculation of the signal as absolute electrons also requires knowledge of the bias offset and the EM Gain. The
calculation path is shown in Figure 35 below:
Figure 35: Count Convert calculation path
Furthermore, knowledge of the Quantum Efciency (QE) at each wavelength, and light throughput properties of the
camera window, enables this process to be taken a step further allowing the signal to be estimated in photons incident
at each pixel. For this step, the user must input the signal wavelength. In uorescence microscopy, for example, this
would correspond to the central wavelength dened by a narrow band emission lter matched to the uorophore
of interest. If the spectral coverage of the signal on the detector is so broad that the QE curve varies signicantly
throughout this range, then the accuracy of the incident photon estimation would be compromised.
The Count Convert functionality of the iXon Ultra provides the exibility to acquire data in either electrons or incident
photons, using both real time and post-process facilities. Since the real time feature is processed in hardware, there is
little or no impact on the display rate. With the post-process option, it is possible to record the original data in counts
and perform this important conversion to either electrons or photons as a post-conversion step, while retaining the
original data.